1. Field of the Invention
The invention relates to a high pressure discharge lamp. The invention relates especially to an ultra-high pressure discharge lamp in which a discharge vessel is filled with at least 0.15 mg/mm3 of mercury, and in which the mercury vapor pressure during operation is at least 150 atm.
2. Description of Related Art
In a projector device of the projection type, there is a demand for illumination of images onto a rectangular screen in a uniform manner and with adequate color rendering. Therefore, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps and more and more often point light sources are being produced and lamps with extremely small distances between the electrodes are being used in practice.
Against this background, instead of metal halide lamps, lamps with an ultra-high mercury vapor pressure, for example, at least 200 bar (roughly 197 atm), have been recently proposed. Here, the broadening of the arc is suppressed by the increased mercury vapor pressure, and at the same time, a great increase of light intensity is the goal.
Recently, by application of a DLP® (Texas Instruments, digital light processor) using a DMD® (Texas Instruments, digital micro mirror device), it is no longer necessary to use a liquid crystal cell. Thus, there has been increasing attention devoted to smaller and smaller projector devices. This means that, in a discharge lamp for a projector device, on the one hand, there is a demand for a high light intensity and a high illuminance maintenance factor, and on the other hand, according to the reduction in size of the projector devices, there is a demand for smaller and smaller discharge lamps. There is also more and more often a demand for more stringent operating conditions.
In a light source device which is used for such a projector device, with respect to projection of clear images, it is considered very disadvantageous that the illuminance of the discharge lamp decreases. This reduction of the illuminance is presumably caused by the fact that the electrode material which has vaporized during lamp operation adheres to the inside wall of the discharge vessel, that in this way blackening of the discharge vessel occurs and that in this way the transmittance of the discharge vessel decreases. As a measure for elimination of the blackening of the discharge vessel, the deposition of tungsten on the inside wall of the discharge vessel is conventionally prevented using the halogen cycle by a halogen which is added to the discharge vessel (see, for example, Japanese patent disclosure documents HEI 2-148561 (U.S. Pat. No. 5,109,181) and HEI 6-52830 (U.S. Pat. No. 5,497,049), hereafter, patent specifications 1 and 2). The term “halogen cycle” is defined as a repeating process in which the metal which has been sprayed off the electrodes reacts with the halogen and oxygen which are present in the discharge vessel, that in this way a metal halide is produced and that afterwards the metal accumulates again on the electrodes.
In the installation of a discharge vessel based on the technology disclosed in the above described patent specifications 1 and 2 in a projector device and in the operation of this lamp it was, however, found that the following two disadvantages arise and that advantageously operation cannot always be carried out.
The first disadvantage is that, during the course of operation of the above described discharge lamp, blackening and milky opacification arise in the discharge vessel with the result that the illuminance maintenance factor is greatly reduced.
Here, it appears that the formation of blackening in the discharge vessel contradicts that above described circumstance. However, blackening of the discharge vessel can only be prevented when that halogen cycle is functioning to an adequate degree. It goes without saying that blackening occurs in the discharge vessel when the halogen cycle is not adequately functioning.
The second disadvantage is that cracks form in the discharge vessel when the above described discharge lamp is turned on and off relatively briefly in succession.